A need exists for cheap, simple, sensitive and rapid nonisotopic immunoassays for the detection of pathogenic viruses, bacteria and cells. Development of an interfacial light scattering immunoassay (ISI) is proposed. Surface immobilized antibodies provide target organism specificity while total internal reflection generated interfacial light scattering provides intense, target specific signal. Specific aims are: (1) Determination of detectability limits, dynamic range and response time for mammalian erythrocytes, streptococcus B and adenovirus, (2) Binding efficacy of monoclonal versus polyclonal antibody will be compared, (3) Three methods of antibody immobilization will be evaluated: (a) passive adsorption on hydrophilic quartz and 14,000 mw poly(ethyleneglycol) (PEG) and on hydrophobic 3-aminopropyl triethoxysilane (APS)-quartz, and covalent bonding to (b) APS- Quartz via glutaraldehyde and (c) Quartz-APS-PEG, (4) Nonspecfic target organism binding and its minimization via PEG surface derivatization will be evaluated, & (5) Design, construct, and evaluate a prototype immunoassay instrument based upon small, disposable, internal reflection element sample plates. Phase I should prove concept feasibility - excellent detection limits for a wide variety and size range of microorganisms. PEG surfaces should minimize nonspecific adsorption of organisms and the prototype should be desirable for use in clinics, hospitals and doctors' offices.